The present invention relates to gray scale test charts for television cameras and more particularly to the gray value configuration in one or more regions of the gray scale test charts.
In television systems including television cameras and television receivers, it is common practice to employ gamma correction or compensation in order to correct for deficiencies of non-linear response in the television receivers, due to transfer characteristics of the picture tube, and thereby produce linear relation between the optical or light intensity input of the camera and the light intensity output at the receiver screen. In any event, this relationship between electrical and light intensity signals is well known in the television art. The problem could be resolved either by adjustment in the camera or receiver. However, it has become common practice in the television industry to make this adjustment in the camera for both black and white or color systems because of the lesser number of units involved. In color systems, the same adjustment is made within each of the three color channels of a television camera. According to industry practice, the electrical signal output of the camera is raised to a predetermined law such as the 0.45 power before it is transmitted to the receiver or before it is stored for subsequent use in the receiver or monitor.
Gray value test charts having one or more regions of known gray values are commonly used for providing a standardized calibration or adjustment in the output of the television camera. One common form for a gray value test chart includes a series of regions or chips having different gray values extending either from relatively absolute black to relatively absolute white or vice versa, the scanning of the gray scale chart by the television camera producing a test signal observed on an oscilloscope or the like which permits manual adjustment of the camera to produce the desired signal increase. Preferably, the gray scale chart includes a series of chips in the form of progressively increasing gray values in inverse relation to the 0.45 law or gamma adjustment desired for the camera. The output signal from the camera, as viewed in the oscilloscope or the like, may then be manually adjusted in order to produce a linear result when the electrical signal output of the camera is raised to the desired 0.45 power law. A common format for the gray value test chart includes one series of gray values progressing from relatively absolute black to relatively absolute white through, for example, a total of nine different chips with a different series of nine chips being physically reversed on the chart and progressing from a relatively absolute white value to a relatively absolute black value.
Practical limits are contemplated for the absolute black values and absolute white values employed in the gray scale chart. For example, it is not possible to reproduce a true jet black having zero reflectance or transmittance. Rather, absolute black is generally considered to comprise approximately 2% reflectance or transmittance. Similarly, absolute white is considered as a practical matter to comprise approximately 60% reflectance or transmittance. These values have been found to be very satisfactory for use in calibrating television cameras since the scenes or objects to be viewed by the cameras are normally included within these relative limits.
In the prior art, gray scale test charts have been produced by manufacturing paint chips which are essentially monochromatic and have varying percentages of light reflectivity or transmittance between the minimum and maximum limits referred to above. The chips are then tested in order to determine the actual presence of any color and to precisely determine the percentage of light reflectivity. Therafter, different sets of these chips are selected to form the stepped gray value regions in the test charts, the chips being secured to the chart as a progressively varying series in the manner referred to above.
This painstaking trial and error method for producing accurate gray value test charts in the prior art has resulted in pigmentation in the various chips produces continuous or uniform substantial expense and problems in reproducibility. For example, reflectance values across the respective gray value regions. Accordingly, as the test charts are exposed to heat and light, the different gray value regions tend to fade or discolor in different amounts, thereby upsetting or undesirably changing the closely regulated progression of gray values in the series of chips. When this occurs, the test chart must be replaced in order to assure continued precise calibration of the television camera.
It is also customary to use photographic transparencies as transmittance charts. These charts are prepared from the same source, for example, and additionally suffer from the non-linear transfer characteristic of the photographic process.
Accordingly, there has been found to regain a need for an accurate and readily reproducible gray scale test chart of either a reflectance or transmittance type, which will preferably be effective for use over extended periods of time.